The present invention relates to a sodium-sulfur secondary battery, and more particularly, to the structure of a cathode chamber of a sodium-sulfur battery having superior charge-discharge cycle characteristics and a simple structure, and a method of manufacturing the same.
The sodium-sulfur secondary battery is one of the sealed type secondary batteries which uses a solid electrolyte tube having a sodium ion conductivity comprising at least either one of .beta.-alumina and .beta."-alumina as an electrolyte, and which separates sodium as an anode active material from at least either one of sulfur and sodium polysulfide as a cathode active material, at least one of which is contained in a battery vessel. Such a battery is capable of being operated at a temperature in the range of 300.degree. C.-350.degree. C. In the present application, and as can be seen in the foregoing, a cathode is defined as an electrode filled with sulfur and/or sodium polysulfide, and an anode is defined as an electrode filled with sodium.
Regarding a sodium-sulfur battery, a technique to place a conductive felt-like carbon mat (rug) material, i.e. an electronic conductor, between the solid electrolyte tube and the cathode vessel, in order to collect electricity of the battery and to maintain the sulfur and sodium polysulfide, i.e. cathode active materials, has been disclosed, for example, in JP-A-6-283201 (1994), and JP-A-7-122294 (1995).
The carbon mat material used in the above mentioned battery was manufactured by perforating needle punches to a web made of flame resistant carbon fiber and calcining the web. However, this technique had problems in that the number of manufacturing steps was large, and the production cost of the mat itself was high, because plural carbon mat materials which had been fabricated in the shape of circular arc needed to be contained in the cathode chamber.
The electronic conductor made of carbon mat material operated as an electric collector between the solid electrolyte tube and the cathode container. Therefore, the contact resistance of the carbon mat material with the solid electrolyte tube was decreased by installing the mat into the cathode chamber in a compressed condition. However, if the compressing force applied to the mat was not uniform, local tensile stresses were generated on the surface of the solid electrolyte tube, and a danger of possible breakage of the solid electrolyte tube could be anticipated after a long period of operation of the battery. Furthermore, when the mat was compressed, a fluctuation in the fiber density was readily generated, and consequently, another problem arose in the form of a fluctuation in the internal resistance of the battery.
In accordance with JP-A-54-109134 (1979), a technique to provide a cathode collector by piling up short carbon fibers and bonding them with a carbide was disclosed. However, the technique had a problem in that the number of steps of the manufacturing process could not be decreased, because a step for piling up short fibers and a step for carbonizing the binder were necessary. Furthermore, fluctuation in electronic conductivity of the binder occurred depending on the carbonizing condition of the binder, and consequently, a problem in which a incrementing of the internal resistance was generated.
JP-A-57-27572 (1982) disclosed a technique to use swelled graphite of 2-50% by weight as a cathode collector. The swelled graphite is graphite which has swelled in a direction of the c-axis among crystalline axes of the graphite. However, in the case of the cathode electric collector disclosed in the above publication, the electricity collecting resistance became higher in comparison with a case when carbon fiber was used as the collector, and consequently, the charge-discharge efficiency was decreased. Several electric collectors have been disclosed in JP-A-61-156640 (1986) and U.S. Pat. No. 4,169,120, in addition to the above references.